Swallow Drama on Great Swamp Day

These swallows were flitting about on some exposed branches, so I took some pictures.  Looking at them, I'm wondering..... what's going on here?  I mean, really, how many reasons are there for a bird to land on top of another bird.......  My guess is - eggs!  Everyone wants eggs!  :-)  But, it doesn't look like they are both the same kind of swallow.  That bottom one doesn't look really very blue.

These pictures were taken all in a row, the camera takes about 5 frames per second.